Project 1 ABSTRACT Studies over the last decade have identified the presence of 3 subsets of blood monocytes. The three subsets are: classical or `inflammatory' monocytes, nonclassical or `patrolling' monocytes, and intermediate monocytes. Elevated numbers of classical monocytes are correlated with progression of human cardiovascular disease (CVD), but less is known about the functions of intermediate and nonclassical monocytes in human CVD. Many published clinical studies have lumped the nonclassical and intermediate human monocytes together, making it impossible to discern their unique functions. Thus, valid functions of these subsets in humans remain elusive. What is clear is that monocytes play key roles in human CVD and that individually targeting different monocyte subsets may modulate the immune-cell dysregulation that occurs in atherosclerosis. Triggering Receptor Expressed On Myeloid Cells-Like 4 (TREML4) is a receptor that is preferentially expressed on myeloid cells. TREML4 recognizes dying and necrotic cells, provides protective immunity, and is required for TLR7 signaling. We found that murine nonclassical monocytes have very high expression of TREML4. Interestingly, a single nucleotide polymorphism (SNP) in TREML4 has been linked with coronary artery calcium (CAC) in two human cohorts. Thus, we propose to study the impact of TREML4 on atherosclerosis progression. We hypothesize that TREML4 is anti-inflammatory and is increased in human and mouse monocytes during atherosclerosis progression to protect against inflammation. The goals of Project 1 are: 1) to test whether TREML4 is atheroprotective, 2) to identify functions of TREML4 on monocyte subsets in atherosclerosis, 3) to test whether TREML4+ monocytes in humans are functionally associated with cardiovascular disease, and 4) to identify how human monocytes are phenotypically and functionally changed in CVD. We will test our hypotheses in the following aims: Specific Aim 1 will test whether TREML4 is atheroprotective. Specific Aim 2 will test how a single nucleotide polymorphism (rs2803496) in human TREML4 known to be linked to CAC functionally changes human monocyte subsets, and how TREML4 is linked to CVD in humans. We will study subjects in the Multi-Ethnic Study of Atherosclerosis (MESA) cohort who have CAC scores =0 (low risk CVD) or >300 (high risk CVD). At the conclusion of our studies, we will know the function of TREML4 in atherosclerosis and if TREML4 is a new biomarker for CAC levels and CVD risk. We will know how human monocytes change during cardiovascular disease, if there are ethnic-specific or gender-specific monocyte changes, and we will have identified new targets for regulating human monocyte subset function in CVD.